madnessario
Knight at Arms
I don't think that necessarily makes them the captain just puts them in that formation (you can assign multiple companions to archer for example). To be a captain I think they need to be leading a party in your army.
CAPTAIN ASSIGNMENTS
Companions can now be assigned to formations from the party screen. Once a battle begins, the companion will be assigned as the formation captain, providing formation-wide perk bonuses to their troops. If the captain dies or flees during battle, the next eligible companion in the same formation will take over as the new captain.